1. Field of the Invention
This invention relates to a finder optical system suited to the single lens reflex type electronic camera of performing still image treatment electrically by using an image pickup element such as CCD.
2. Description of the Related Art
In the past, the single lens reflex camera using the pentagonal roof prism for silver halide film of 35 mm size has greatly developed as is optimum on making the system to expand. The construction of its typical finder optical system is schematically shown in FIG. 6. In FIG. 6, 101 is a total-reflection mirror; 102 is a shutter unit; 103 is a film plane; 104 is a focusing screen; 105 is a pentagonal roof prism: 106 is an eyepiece lens; and 107 is a pupil for observation. The finder optical system shown in FIG. 6 has excellent optical performances that the ratio of the object image observed by the finder optical system to the image area photographed on the film plane, i.e., the viewfield rate, is more than 90% and the viewfield magnification .gamma. is more than 0.8.times. when the standard lens is in use. And, this finder optical system has a feature that the whole apparatus can be constructed with a relatively small size.
However, when the above-described pentagonal roof prism is used in a finder optical system for the electronic camera using a plurality of image pickup elements such as the so-called CCDs, namely, of the 2-plate type, or of the 3-plate type, it becomes difficult to obtain as high a viewfield rate and as high a viewfield magnification as in the conventional single lens reflex camera. This owes to the following reasons:
(i) Since the effective image area of, for example, the 2/3 in. image pickup element is small as compared with the 35 mm film, being 1/4 in diagonal length ratio, if the conventional pentagonal roof prism is employed, the optical length is too long to obtain a high viewfield rate and a high viewfield magnification without difficulty.
(ii) Since a large space is needed for arranging the electrical processing circuits in the rear of the image pickup element, the distance from the image plane of the photographic lens to the rearmost end of the camera becomes long. For this reason, the construction must be made such that the pupil position of the finder optical system is extended to the camera side rear. This results in the difficulty of obtaining the high viewfield rate and the high viewfield magnification.
(iii) Since in front of the image pickup element, many spaces are needed for arranging the separation prism for splitting light to a plurality of light beams, the low pass filter, the infrared cut filter, the protection glass, etc. a large distance between the split point of the finder optical path and the image pickup plane must be taken, causing the whole apparatus to become large.
Next, for reference, one example of a finder optical system of the case of aiming that a higher viewfield rate than 90% is achieved by using the conventional pentagonal roof prism in the electronic camera is shown in FIG. 7. In FIG. 7, 200 is a photographic lens; 201 is a splitting unit for splitting the optical path from the photographic optical path to the finder optical system; 202 is a low pass filter; 203 is a shutter unit; 204 is an image pickup plane of an image pickup element; 205 is a package of the image pickup element with a protection glass having the infrared cut effect arranged in front thereof; 206 is a finder optical system unit including an erecting non-reverse image system; 207 is a focusing screen; 208 is an electrical processing circuit unit for image pickup signals; 209 is a pupil for observation. Also, 204' is another image pickup plane. 210 and 211 are separation prisms for image separation of a photographic light beam.
In general, the greater the viewfield magnification .gamma., the easier the finder image becomes to observe. The viewfield magnification .gamma. is expressed by .gamma.=f.theta./fe where f.theta. is the standard focal length of the photographic lens and fe is the focal length of the eyepiece lens. To increase the viewfield magnification .gamma., because the focal length f.theta. of the photographic lens is almost constant, the focal length fe of the eyepiece lens must be decreased. Since the eyepiece lens is so arranged that its front focal point lies at or near the finder image plane of the finder optical system, for an increase of the viewfield magnification .gamma., the optical path of an optical system for obtaining an erect non-reverse image from the focusing screen to the eyepiece lens must be made as short as possible.
On the contradictory, in the case of the electronic camera, as has been described before, on the front and rear sides of the image pickup plane of the photographic system there must be arranged the infrared cut filter and the low pass filter and further the electrical processing circuit for electrically processing the image information and other units. For this reason, there is need to shift the movable mirror (quick return mirror) for conducting the light beam from the object to the finder optical system toward the object side (front), and also the observing position toward the rear.
Therefore, in the case of the arrangement that the image for the finder of the photographic lens on the focusing screen is directly observed through the eyepiece lens, it is unavoidable to increase the focal length of the eyepiece lens with the result that the viewfinder magnification becomes small. Hence, the finder becomes harder to observe. And, while attempts have been made to reduce the focal length of the eyepiece lens as proposed in, for example, Japanese Laid-Open Patent Application No. Sho 60-43628, the finder magnification has nevertheless been limited to about 0.5.times..
Meanwhile, the latitude of the electronic camera using the solid state image pickup element such as CCD is narrow compared with the camera using silver halide film, being about 1/2-1/3 times that of the silver halide film.
Therefore, the accuracy of light measurement must be higher than that for the still camera using silver halide film. And, as the general arrangement of the light measuring element in the camera using silver halide film there are known the side portion of the eyepiece lens and the bottom plate of the camera body. In any case, however, by the influence of the squint, etc., it was difficult to carry out light measurement without the squint accurately.
Further, again in the case of the electronic camera using supplemental optical members such as the beam splitting prism for splitting the light beam, the low pass filter and the infrared cut filter, the image quality of the image pickup element formed with the light passing this prism and the image quality formed with the light not passing the prism come to differ from each other by an amount corresponding to the optical path of the prism. In a case where the focus detecting device is merely put in the position to which the split light beam in passing through the quick return mirror is directed by using a sub-mirror provided thereon and before which there are no supplemental optical members, because the image qualities of the image pickup element and the focus detecting device come to differ, the accuracy of distance measurement is difficult to improve. Particularly in the electronic camera whose image plane size is smaller than that for the silver halide film, the degree of allowance of the field depth becomes smaller. Hence, a higher accuracy of distance measurement is required.